How long does it take for a lens protocol post to index?

It gets indexed almost immediately, however, you need to make sure you have all the required metadata in your content.

You can test by running a metadata query on api.lens.dev:

query ValidatePublicationMetadata {
  validatePublicationMetadata(
    request: {
      metadatav2: {
        description: "What are stablecoins? What do they do in DeFi? Why do people mint stablecoins? Where are stablecoins as if Nov 2022?"
        content: "Your article here"
        name: "What Is a Stablecoin? (But Actually)"
        image: "ipfs://QmQBxgAbbHqZMTQYyGE44siuZ6BzQGeHt3vGHCENpj3TEv"
        version: "2.0.0"
        metadata_id: "ad436b9e-2cbb-4140-b199-368533c4b88d"
        locale: "en-US"
        mainContentFocus: ARTICLE
        imageMimeType: "image/png"
        appId: "lensBlog"
        attributes: []
      }
    }
  ) {
    valid
    reason
  }
}